Organophosphate (OPs) compounds are widely used in intensive agriculture to improve production, protect crops and control diseases vectors. The main mechanism of toxicity of OPs is the inhibition of the Cholinesterase enzymes. In this study the acetylcholinestrase (AChE), butyrylcholinesterase (BChE) and specific acetylcholinestrase (SAChE) activities as potential biomarkers of exposure to OPs were evaluated in whole blood and plasma samples of exposed farm workers of Shushtar and Dasht-e Azadegan in Khuzestan, the south-west province of Iran based on the modified Ellman colorimetric method. Then, RBCs (Red Blood cell) and plasma cholinesterases, haemoglobin (Hb) and specific activity of acetylcholinestrase in Shushtar and Dasht-e Azadegan farm workers were compared with the control group.Results obtained in this study showed that the means of RBC AChE activities in Dasht-e Azadegan, the BChE activities in Shushtar as well as the SAChE activities in both groups were significantly (p < 0.05) lower than their means of activities in their control group. Also, means of haemoglobin concentration in samples obtained from Shushtar (p: 0.016) and Dasht-e Azadegan (p: 3×10-4) were significantly (p < 0.05) higher than the control group.Data obtained in this study indicated that cholinesterase enzymes inhibition provides a good biomarker of exposure to organophosphate pesticides in field studies of human population.